384 PROF. J. W. JUDD OiS^ THE PROPYLITES 



Plate XV. 



In this Plate an attempt has been made to illustrate some of the varieties of 

 the Pyroxene-andesite of Ben Hiant. In these rocks the minerals and glass 

 composing the difl'erent masses are identical ; but the proportions in which thej' 

 are combined and their structural relations are so different as to give rise to 

 some very strikingly contrasted rock-tvpes. They are shown as seen magnified 

 25 diameters. (See pp. 373-380.) 



Fig. 1 is a compact Augite-andesite from the south side of the mountain, and 

 under the microscope is seen to be a " microlitic felt," consisting of 

 lath-shaped felspars, with granules of augite and magnetite, imbed- 

 ded in a glassy base. The rock contains numerous small vesicles, 

 which are sometimes filled with glass, as described by Osann and 

 Teall. (See p. 378.) 



Pig. 2 is a rock with a similar ground-mass, through which numerous porphy- 

 ritic crystals of plagioclase, Avith some of augite and magnetite, are 

 scattered. The plagioclase crystals contain many glass- and magne- 

 tite-inclusions ; and in the gi-oimd-mass there are a few vesicles 

 filled with secondary products. In all its essential characters the 

 rock is quite imdistinguishable from the Augite-andesites of Hun- 

 gary. (See p. 377.) 



Fig. 3. " Pitchstone-porphyry " from the east side of the mountain (at the 

 point marked # on the map and section). In this rock the quantity 

 of glassy base becomes very large, and the crystals of felspar, augite, 

 and magnetite occur in sparsely scattered groups. The felspar- 

 crystals are sometimes much corroded. The brown glass of this 

 rock is traversed by many cracks, showing a distinct approximation 

 to a perlitic arrangement. (See p. 378.) 



Fig. 4. Augite-andesite with portions of glass full of magnetite needles caught 

 up between the numerous crystals, giving rise to the " intersertal "' 

 structure of Professor Eosenbusch. (See p. 378.) 



Fig. 5. Augite-andesite in which the " intersertal " structure of the last type is 

 combined with the ophitic structure. (See p. 378.) 



Fig. 6. Kock differing from the last by the almost complete disappearance of 

 the glassy material, so that the ophitic structure dominates through- 

 out the whole mass. (See p. 378.) 



Fig. 7 is a rock in which a glassy base (with a few vesicles in it) encloses nmue- 

 rous large crystals of labradorite (or of a felspar near that species). 

 Augite and magnetite are present only in comparatively small quan- 

 tity and as minute individual granules in the ground-mass, and the 

 whole rock becomes a^ good example of a " Labradorite-andesite." 

 (See p. 378.) 



Fig. 8 forms a striking contrast to the last. There is little glassy matter and 

 the felspar is present in small proportions. The bulk of the rock 

 consists of augite and magnetite, and, though so closely related with 

 the other types, is remarkable for its low silica-percentage and high 

 density. (See p. 378.') 



Discussion. 



The President said that papers like the present were difficult to 

 discuss. They required to be read and reflected upon. 



Mr. Barrow, referring to the amphibole- and pyroxene-andesiest, 

 remarked that, in mapping- some of the dykes in Scotland, he had 

 come across cases where amphibole prevailed in an acid matrix ; 

 secondly, where tlie prevailing mineral was mica ; thirdly, 

 pyroxene ; and fourthly, that the three minerals would occur 

 together. All these dykes were parallel to one another, and often 

 formed double dykes with the apophyses of the Dee-Side granite. 



